This invention relates to the control of particulate matter and the residual gas content of flue gases, and, more particularly, to a control system for the addition of conditioning agents to the flue gas.
In a coal-fired power plant, coal is burned to heat air, which in turn boils water to form steam. The steam drives a turbine and thence an electric generator, producing electricity. Besides heat, the burning of the coal produces gaseous pollutants such as sulfur and nitrogen oxides, and a solid particulate known as fly ash. Environmental protection laws mandate that the amounts of gaseous pollutants and solid particulate emitted from the power plant be maintained at acceptably low levels, and the present invention deals generally with the technology for controlling particulate emissions.
One widely used approach for removing the particulate fly ash from combustion or flue gas streams is electrostatic precipitation. The flue gas stream with entrained particulate is passed between highly charged electrodes that ionize the particles so that they are attracted to, and deposited upon, a collection electrode. The particulate may optionally be charged prior to entry into the precipitator to increase the efficiency of removal. The cleaned combustion gases are released to the atmosphere, and the precipitated particulate is removed from the collection electrode.
The efficiency of operation of electrostatic precipitators depends markedly upon the electrical resistivity of the particulate. If the resistivity of the particulate is too high, the ability of the precipitator to collect particulate is greatly reduced due to reactions, such as a back discharge, that interfere with collection. Consequently, an unacceptably high portion of the particulate is exhausted to the atmosphere. There exist conditioning procedures and apparatus for altering the electrical resistivity of the particulate by injection of conditioning agents into the combustion gas stream prior to its entering the electrostatic precipitator.
An example of such a treatment procedure is that disclosed in U.S. Pat. No. 3,993,429, and this approach has become widely accepted and used throughout the United States and the world. In this approach, a conditioning gas such as sulfur trioxide is injected into the combustion gas stream. The sulfur trioxide reacts with moisture in the gas stream to produce sulfuric acid that is deposited upon the surface of the particulate. The sulfuric acid reduces the electrical resistance of the particulate, which is equivalent to raising the electrical conductivity of the fly ash particulate, so that the electrostatic precipitation treatment works well. Conditioning treatments are routinely used where the sulfur content of the coal burned in the power plant is so low that the electrical resistivity of the resulting particulate is too high to permit the electrostatic precipitators to operate properly.
It is also known to add other conditioning agents such as ammonia to the flue gas. The ammonia reduces the amount of residual sulfur trioxide in the flue gas by forming ammonium sulfates and bisulfates. The ammonium bisulfates have the added beneficial effect of acting as a binder of the deposited fly ash in the electrostatic precipitator, so that there is a decreased likelihood that deposited fly ash can become reentrained in the gas stream to be exhausted through the stack and into the atmosphere.
Although the combination of sulfur trioxide and ammonia conditioning agents is currently used in some power plants to improve the quality of the gas stream emitted to the atmosphere, it is difficult to determine the correct amount of each conditioning agent to add to the flue gas stream to achieve optimal results, under the wide variety of operating conditions possible even in a single combustor and cleanup system. If relatively too much sulfur trioxide is added, there may be residual sulfur trioxide in the gas released to the atmosphere, a cause of acid rain. If relatively too little sulfur trioxide is added, the electrostatic precipitator does not operate at its highest collection efficiency, and particulate is released in the atmosphere.
The amount of conditioning agents added to the flue gas should be selected to optimize the system performance, for both gaseous and particulate emissions. At the present time, the control procedures are performed largely manually, based upon the observations of the operator. There exists a need for an improved approach to controlling the addition of conditioning agents to the stream of flue gas, so that the adjustments may be done automatically and reproducibly under a variety of operating conditions. The present invention fulfills this need, and further provides related advantages.